Monday 6th July
After a cool, cloudy start to Sunday the sun began to break through late morning and built after midday to reach a maximum of 27.2C at 15.59, being 4.3C above my long-term average. The heatwave threshold for Wiltshire of 27C was reached at 15.12 and lasted for an hour until 16.14 when variable light cloud drifted across resulting in the peak temperature beginning to slowly decline. A minimum of 13.8C was logged at 04.50 early Monday, just before sunrise in Marlborough at 04.58, being 1.8C above average.
In the early hours of Monday low cloud had drifted across that initially masked the top of the Marlborough Downs giving murky conditions but by 06.15 this had completely evaporated. The thin, high cloud will likely begin to recede as the morning progresses and the heat to build again.
Sunday was the start of the third heatwave of 2026. There is no doubt that an official heatwave will be declared on Tuesday when there will have been three successive days when the temperature reached or exceeded the Wiltshire threshold of 27C. In fact it is likely that this heatwave will exceed the seven days of the second heatwave in June, which will probably last up to the middle of next week when the temperatures are then forecast to moderate a little but still be well above average.
Although peak temperatures by Wednesday will likely reach 30C of above, the nights won’t be quite so hot as in the previous heatwave. However, they will still be a few degrees above average for July but thankfully no tropical nights are likely when the thermometer does not fall below 20C.
The Met Office heatwave threshold was updated ahead of summer 2022. The initial heatwave thresholds were calculated based on the 1981-2010 climatology of daily maximum temperature at the mid-point of the meteorological summer (15 July). The revised thresholds use the 1991-2020 averaging period introduced in January 2022. The geographical differences reflect the differences in climate across the UK.
Over the last few years there has been a push to install solar panels. While the panels work best in direct sunlight, they ironically don’t do as well in high temperatures – when the sun is at its strongest – due to how they generate electricity. It is not widely known that these panels begin to loose their optimum performance when the air temperature reaches around 25C and for every degree Celsius above that temperature the efficiency falls by around by 0.5%. More recent panels can cope better than earlier versions but still see a drop off in performance in heatwave conditions.
Latest: Heatwave threshold of 27C reached at 12.01